(i) Current conventional wisdom is that direct firing of coal or the use of hot, substantially uncooled gas from a coal gasifier is probably the best means to achieve high generation efficiencies with coal-fired gas turbine systems. However, it is known that ash particles, vaporised salts, and sulphur compounds which result from direct coal firing or from the use of hot gases from coal gasifiers, are difficult to eliminate without substantial cooling of the gases.
(ii) It is known that cooling and washing of gas leaving coal gasifiers can substantially remove all the compounds which give rise to problems with high temperature components in gas turbines. However it is accepted that such a cooling and washing produces a very dilute fuel gas heavily contaminated with water vapour which is generally regarded as being unsuitable for combustion in conventional gasifiers. In some currently proposed schemes for Integrated Gasification Combined Cycle processes (known in the art as IGCC processes), fuel gas produced by simple total or partial adiabatic cooling of gasifier product gas also lowers the flame temperature of the diluted fuel gas to the point where stable combustion and the high combustion temperatures required by high efficiency IGCC system designs are not attainable with the highly water vapour-diluted coal-based fuel gases.
(iii) It is generally accepted that, notwithstanding the known problems of added complexity caused by the introduction of additional equipment, efficient coal firing of gas turbines can only be achieved with the use of combined cycles, ie. the incorporation of a waste heat boiler in the turbine exhaust and the use of a steam turbine cycle.
(iv) It is also conventional wisdom to consider that higher compression ratio turbines are the optimum means to achieve high efficiency with coal-fired gas turbine systems even though such turbines present known problems associated with the use of high compression ratios. The problems of drying and gasification of carbonaceous fuels and particularly high water content carbonaceous fuels such as lignite, when used as a power generation fuel or feedstock for synthesis gas production, are also well known.
(v) Prior Art Proposal
The foregoing problems are outlined in a recent prior art development which proposes a means and process whereby solid moisture-containing coal can be utilised to generate power. According to this proposal, the process cannot however readily deal with a solid lignite having a significant moisture content if it is converted into a water-lignite slurry typically containing 25% or possibly less solids (wet basis). Even though the higher water content lignite could be more readily handled, it was apparently considered that loss of efficiency of the overall process and also the known problems in combusting water vapour laden, very low heating value fuel gas in gas turbines would be too disadvantageous. Consequently, the use of such a coal slurry was apparently considered to be not technically feasible and to be likely to result in major combustion problems, and inability to achieve a high combustion temperature with consequent low power generation efficiency.
In this (solid fuel) proposal, the fuel gas leaving the drier integrated with the gasification process is to be further adiabatically cooled by the injection of water and the saturated gas is then washed or treated by known means to remove particulates, and is then further cooled to reduce the water content of the gas such that it is increased in calorific value and can be used in known combustion systems. Gas treatment at water saturation temperature can remove particulates and may also enable the removal of sulphur compounds by known means following that stage. This final cooling has the inherent disadvantage of removing water vapour at combustion pressure which could be heated in the combustion stage and a significant part of its inherent energy recovered in the expansion stage of the turbine. By condensing the water vapour this inherent energy is wasted and the cooling stage requires extra equipment and cooling media to remove the water vapour and a process plant to handle the resultant condensate. PA1 The cooling of the gasifier exit gas by simultaneous drying of the coal feed puts additional water vapour into the fuel gas, and further adiabatic cooling by water addition and water washing for removal of solid particles and possibly sulphur compounds after the drying stage puts water vapour at combustion stage pressure (which could increase the mass flow to the combustion and final expansion stage thereby increasing the rated power output of the turbine system). However the additional water content added by such cooling has the problem of making the gas unsuitable for combustion and for the achievement of high combustion temperatures such as in excess of 1,100.degree. C. and preferably above 1,200.degree. C. PA1 It is also known, and referred to in this prior art, that it has been proposed that wastes such as sewerage sludge can be dried by contacting said wastes with the hot gases leaving a coal gasifier. However such a system also suffers from the limitation that the amount of water which can be evaporated is limited by known gas turbine and turbine combustion systems. These limitations also constrain the use of simple water spray and gasifier exit gas cooling and ash removal systems. PA1 The problems of NOX (oxides of nitrogen) formation in fossil fuel fired gas turbines are well known. The prior art proposal envisages the use of conventional known combustion systems which, even with best practice combustor design, would result in a NOX content in the exhaust gas of in excess of 10 ppm (and probably about or in excess of 20 ppm). PA1 (i) coal, in the form of an aqueous slurry having a water content of at least 55% by weight, is introduced, with hot gas produced in a coal gasifier stage, to a drying stage, PA1 (ii) the resultant slurry mixture is dried in the said drying stage by the adiabatic cooling of the hot gas and evaporation of the water, the thus dried coal and the cooled humidified gas being separated with return of the dried coal to the hot-gas producing gasifier stage, and PA1 (iii) the cooled humidified gas is further cooled, cleaned and utilised in the production of chemicals and/or electrical energy. PA1 The lignite/brown coal/high water content semi bituminous coal being fed to the drying stage of the process is fed by pumping, or other known means, as a slurry or paste of size reduced, but otherwise untreated "as mined" lignite or coal. This feed material has a water content in the range 70-80% by weight of water. PA1 Combustor system for the turbine comprises two combustion stages, the first stage being a combustor in which sufficient fuel gas is intimately mixed with the combustion air prior to the first stage of combustion to give a temperature in excess of 800.degree. C. and below 1,000.degree. C. at which temperature the mixture leaves the first stage. The remaining part of the fuel (as required) is added under conditions which ensure maximum mixing and turbulence such that free radical induced rather than flame induced combustion is favoured. An example of such a combustor is described in the specification of application PCT/AU95/00719 which is incorporated herein by reference. PA1 The fuel gas passing to the second stage of combustion is catalytically reacted (e.g. per medium of known sulphur tolerant shift catalysts) to convert at least part of the carbon monoxide and water vapour in the gas stream to hydrogen and carbon dioxide before passing to the said second combustion stage. This further increases its temperature and hydrogen content to ease its combustion in lean phase mix with combustion gases by means of free radicle-induced combustion. PA1 The gases leaving the coal gasification stage, or at least the greater part thereof, are used to dry the coal (lignite) water slurry and the gases are then further cooled by evaporative cooling by further water addition, water washing being thereby used to remove solid particles from the gas (by known means such as venturi scrubbers, water sprays, coalescers, demisters, electrostatic precipitators). Thereafter the gases can be preheated by heat exchange with exhaust gases leaving the expansion stage of the gas turbine before being used as fuel gas in the turbine's combustion system. PA1 Salts may be removed by solution in the wash water and fuel and ash components may be separated by known means. Unused fuel may be recovered as a water/fuel slurry and recycled with the incoming fuel/water slurry feed. PA1 The water content in the coal may be due in whole or in part to the addition of wastes such as sewerage waste or other suitable wastes requiring disposal which may be added together with the coal or lignite fuel.
Further Known Problems
(vi) It is known that oxygen should preferably be used for coal gasification where the product gas is to be used to synthesise methanol or methanol derivatives. However for such gasification processes, the reduced flow of produced gas compared to coal feed rate makes integrated drying and gasification difficult. It has been believed that, due to lack of sufficient hot gas needed to dry the coal prior to gasification, the use of oxygen-blown gasifier gas to dry a significantly wet lignite feed cannot be achieved.
(vii) It is also known that gasification processes developed for bituminous coal--such as the processes commercially known as U-Gas and Shell Totzec which operate at pressure, require a dry coal feed and therefore cannot use a pumped water/coal slurry--require the use of lock hopper systems to pressurise the coal prior to being fed to the gasifier. The gas mixture in the gasifier is flammable and toxic and such feed systems require special equipment to recover and use these gases.
(viii) A further known problem with IGCC cycles, particularly air blown gasifier processes, is that, to enable conventional burner systems to operate satisfactorily, it is necessary to operate the gasifier at a significantly higher pressure than the pressure to which the combustion air is compressed.